1. Field of the Invention
The present invention relates to apparatus for measuring transparent aspheric surfaces. Examples of such surfaces include the cornea of the eye and the surface of a contact lens. More particularly, the invention combines an optical system with a video image analysis system to generate a mathematical expression describing the shape of a transparent aspheric surface.
2. Description of the Art
In the prior art it has been recognized that accurate characterization of the shape of the surface of the cornea would aid in the fitting of contact lenses. Initially it was believed that the cornea had a substantially spheric shape and that the "fit" between the cornea and the contact lens need not be exact, for the comfort of the wearer or for operation of the lens. In general, it was believed that "soft" contact lens material would be compliant enough to conform to the corneal shape, and that only a few "base curves" would suffice to fit the majority of the population. Lindmark et al. demonstrated that the corneal shape is not spherical and that failure to accommodate the complex surface of the cornea can result in injury to the cornea itself, see "The Correction of Atypical Ametropia with Flexlens", Vol. 13, 1979, of the Contact Lens Journal, R. C. Lindmark, et al.
In the prior art, there have been two principal techniques used for estimating the "sphericity" of the cornea. The earliest systems involve the projection of Placido's rings onto a cornea. The practitioner observes the clarity and spacing of the projected rings and compares the resultant image pattern with reference curves to estimate which one of a collection of spherical curves most closely approximates the surface of the cornea. The earliest systems of this type relied on direct observation of the projected rings on the eye. More recent versions of this system photograph the ring pattern on the eye producing a karotograph which may be evaluated with the use of a computer system. Examples of this approach are taught by U.S. Pat. No. 4,685,140 to Mount; U.S. Pat. No. 4,978,213 to El Hage; In each of these systems a television camera is utilized along with a computer to process the Placido's ring data.
An alternate approach develops an image of the cross section image of the meridian of cornea of the eye with an optical system. This approach is typified by the Corneoptor system developed by Scientific Advances Incorporated of Columbus, Ohio. in the late 1960's. This system utilizes a slit illuminator to develop a photograph of the cross section of the cornea. In use the operator would compare the photographed image with a set of test curves to determine the "best fit" representation of the contour.
It is now well recognized that the cornea of the eye has a very complex aspheric shape, and that more exact knowledge of this shape for particular individuals would be an aid to fitting contact lenses. Such a system would also be useful for evaluating the corneal surface for surgical procedures and for evaluating the base curve and front curve of contact lenses as an aid to fitting them on the human eye.